Sight devices are commonly used with firearms to provide a shooter with an aiming point. Several types of sight devices are available. For example, “iron” sights typically include a first sight piece positioned near the muzzle end of a firearm and a second sight piece positioned nearer to the breach end. The first and second sight pieces are positioned appropriately with respect to one another to align the firearm with an object.
Optical sights are another type of sight device and include optical components, such as lenses, and an indication of an aiming point. Typically, this indication of an aiming point is in the form of a reticle, which can have many configurations, such as dots, crosshairs, and others. Telescopic sights are a type of optical sight and include an optical system that magnifies the image viewed through the telescopic sight.
For some telescopic sights, the components of the optical system are essentially fixed with respect to each other and provide little or no adjustment. For other telescopic sights, some components of the optical system may be moved to make adjustments in the image viewed through the telescopic sight. Particularly, the position of one or more lenses may be changed to adjust the amount of magnification of the viewed image. The positional relationship between the components of the optical system is very sensitive to even fine adjustments, however, and making adjustments, such as to change the magnification or to focus on objects at various distances, can introduce parallax into the optical system.
Parallax is evident when the point of aim viewed through the telescopic sight changes as a shooter moves his eye laterally or vertically with respect to the telescopic sight. Where a telescopic sight includes a reticle as the indication of the aiming point, such an optical component is known as a reticle piece. In the absence of parallax, the point of aim indicated by the reticle does not move when the shooter moves his eye. Parallax is caused by a very slight misalignment of components in the telescopic sight. Particularly, parallax is caused when an image of the object is not focused in the same focal plane as a component that carries the indication of the aiming point.
Many telescopic sights have therefore been provided with an adjustment feature for removing parallax. In particular, such optical sights include either a front (adjustable objective) or side mounted parallax adjuster, and the parallax adjuster is used to cause a change in the alignment of the components of the optical system to remove parallax and bring the image of an object into focus in the same plane as the reticle piece.
Some parallax adjusters have included marks that attempt to identify an adjustment position to eliminate parallax for a particular distance to a target object. For example, an adjustment dial could have marks for 50 yards, 75 yards, 100 yards, 150 yards, and so on. If the shooter knows a distance to an object, say 100 yards, the shooter would operate the parallax adjuster to the position associated with the 100 yard mark to remove the parallax, at least in theory. The longest adjustment setting, usually representing a distance beyond the effective shooting range of the rifle is typically designated as “infinity” (∞).
Such marks have proven to be less than ideal for reliably and accurately removing parallax, however. Particularly, it is believed that environmental conditions, especially temperature, can influence a telescopic sight in a manner that makes such marks only approximate guides, at best. For example, operating a parallax adjuster to the same adjustment position might have different effects depending on whether the environmental conditions of the telescopic sight are hotter or colder. In other words, an adjustment position that corresponds with no parallax at a given distance at a first temperature might not remove parallax at a second temperature. Experimentation has shown that a change in temperature as little as 2° F. can significantly affect the parallax setting. It is believed that, in high magnification precision optics, thermal expansion/contraction of the scope body relative to the factory temperature is enough to affect the axial spacing of the optical elements to an appreciable degree. Whether in hunting, military or tactical settings, it may be expected that the temperatures at which a rifle scope may used to span a range of 100° F. or more.
In addition to helping a shooter identify an adjustment position for removing parallax, such marks could also be helpful for shooters attempting to determine the distance to an object. For example, if a parallax adjuster were used to remove parallax, the corresponding adjustment position, as indicated by a mark, could identify the distance to the object. However, if the marks do not accurately reflect the actual distance to an object, this approach is unlikely to provide useful information. Moreover, because these indictors cannot be user-corrected for temperature or other variations, the markings tend to be either unlabeled (and, therefore, representing arbitrary or unknown increments) or labeled in very coarse and imprecise increments of 100 yards (or meters).
Scopes with higher magnification are generally intended to be used for aiming at longer distances. Because the range of parallax adjustment is limited, the adjustment mechanism usually does not allow for adjustment for short ranges, such as less than 100 or 50 yards (or meters).
There is a need, therefore, for telescopic sights that address one or more of the challenges discussed above.